The present invention relates to fluorosilicone rubber compositions. More particularly, the present invention relates to additives which increase the heat stability of fluorosilicone rubber compositions.
Fluorosilicone rubbers are useful in the preparation of hoses, seals, gaskets, and linings for fuel tanks and for electrical insulation which is subjected to fuels, lubricating oils and hydraulic fluids. These applications require such rubbers to possess certain physical properties, such as high tear strength, high elongation, and low compression set. In addition to the foregoing properties, fluorosilicone rubber compositions used in the applications described above must also be heat stable.
Silicone rubbers are generally stable at temperatures below about 150.degree. C. and may be used at temperatures up to about 260.degree. C. and even up to 315.degree. C. for short periods of time. However, at temperatures above about 200.degree. C. for extended periods of time, silicone rubbers tend to deteriorate rapidly. Fluorosilicone rubbers are even less heat stable than their non-fluoro-containing silicone rubber analogs because the fluorosilicone rubbers are more susceptible to siloxane bond cleavage and oxidative degradation. Since fluorosilicone rubbers are generally used at elevated temperatures in many of their applications, deterioration at such temperatures is highly undesirable.
It is known in the art that amphoteric metal oxides and hydroxides inhibit the oxidation and decomposition of fluorosilicone rubber over the range of 250.degree.-300.degree. C. Reference is made, for example, to E. L. Warrick, et al., Rubber Chemistry and Technology 52,442-443 (1979).
U.S. Pat. No. 3,647,741 (Hutchison) discloses the use of fumed titanium dioxide to improve the heat stability and compression set of silicone elastomers.
U.S. Pat. No. 4,355,121 (Evans) teaches the use of a rare earth octoate as a heat stabilizer.
However, while amphoteric metal oxides and hydroxides provide a fluorosilicone rubber with adequate resistance to heat aging, such compounds adversely affect certain of the rubber's physical properties, e.g., hardness, elongation, and tear strength. With these compounds, the fluorosilicone rubber's Durometer hardness increases while the elongation and tear strength decrease.
It is desirable therefore to provide a method for improving the heat stability of a heat curable fluorosilicone rubber composition without sacrificing the physical properties of the rubber composition.
Such a method, as well as the improved heat curable fluorosilicone composition resulting therefrom, is provided by the present invention.
The present invention is based on the discovery that a combination of red iron oxide and cerium hydroxide, each in critical amounts, will enhance a fluorosilicone rubber's resistance to thermal aging at elevated temperatures, e.g., 200.degree. C. and higher, without sacrificing the rubber's physical properties.
U.S. Pat. No. 4,525,528 to Bush et al. (Bush) discloses the use of iron oxide as a heat aging additive and the use of cerium hydroxide as a compression set resistance promoter in a fluorosilicone composition. However, the Bush patent does not teach that critical amounts of cerium hydroxide and iron oxide will increase the heat stability of a fluorosilicone composition.
The combination of red iron oxide and cerium hydroxide in critical amounts has a synergistic effect on the heat stability of the fluorosilicone rubber in that the combination of the two ingredients will provide superior heat stability than the use of either component alone or in combination with other heat age additives.